This invention relates generally to the formation of thin films, and, more particularly to the formation of thin dielectric films by gas-cluster ion-beam (GCIB) on surfaces rendered free of native oxides to assure high quality interfacial layers and films.
The use of a GCIB for etching, cleaning, and smoothing of the surfaces of various materials is known in the art (See for example, U.S. Pat. No. 5,814,194, Deguchi, et al., xe2x80x9cSubstrate Surface Treatment Methodxe2x80x9d, 1998). Means for creation of and acceleration of such GCIBs are also described in the Deguchi reference. It is also known (U.S. Pat. No. 5,459,326, Yamada, xe2x80x9cMethod for Surface Treatment with Extra-Low-Speed Ion Beamxe2x80x9d, 1995) that atoms in a cluster ion are not individually energetic enough (on the order of a few electron volts) to significantly penetrate a surface to cause the residual sub-surface damage typically associated with the other types of ion beam processing in which individual ions may have energies on the order of thousands of electron volts. Nevertheless, the cluster ions themselves can be made sufficiently energetic (some thousands of electron volts), to effectively etch, smooth or clean surfaces as shown by Yamada and Matsuo (in xe2x80x9cCluster ion beam processingxe2x80x9d, Matl. Science in Semiconductor Processing I, (1998) pp 27-41).
The heart of high-density memory and microprocessor chips is a very thin film of an electrically insulating material formed on the surface of a silicon crystal wafer. This insulator, referred to as the gate dielectric, must sustain very high electric fields and serve efficiently as the key component in the storage of electrical charge. A conductor film, not necessarily a true metal, must then be formed on top of the dielectric. Two basic types of microelectronic devices are fabricated from these so-called metal/insulator/silicon (MIS) layered structures, namely bit storage or xe2x80x9cmemoryxe2x80x9d, and logic transistors. An assembly of many bit-storage units on a single silicon wafer is used to fabricate dynamic random-access memory (DRAM) integrated circuits, while an assembly of many logic transistors is used to fabricate microprocessors. As the chip industry moves toward smaller circuit elements packed more tightly on the chip, it is required that the area devoted to each charge-storage circuit on the chip be not only smaller but also store at least the same amount of charge. Two approaches are possible, first the thickness of the gate dielectric film can be reduced and second the intrinsic storage ability of that insulator, called its dielectric constant, can be increased by choice of a new kind of material. Up until the present time, the chip industry has used silicon oxide (nominally SiO2, referred to as silica or just xe2x80x9coxidexe2x80x9d) as the dielectric and has succeeded in making each chip generation with thinner oxide films. When the insulator is this oxide, then the structures consisting of a conductor (metal or polysilicon) film, on an oxide film, in turn on silicon, is the metal/oxide/silicon (MOS) structure employed as the basic building block unit of the vast majority of the semiconductor industry product. But the trend of continuing to reduce the thickness of the oxide film in the MOS is projected to run out of potential as it reaches basic physical limits.
Silicon oxide thin films have been the basis for gate dielectrics in silicon-based very large scale integration (VLSI) complementary metal-oxide semiconductors (CMOS) for several decades now. As the industry moves toward more advanced devices, the gate dielectric thickness is reduced with each design generation but will soon reach what is believed to be an ultimate limit. When the physical thickness of any dielectric material is less than about 5 to 15 xc3x85, direct quantum tunneling across this dielectric barrier results in sufficient current (leakage) as to cause the CMOS transistors to malfunction. CMOS now being developed for production manufacture and marketing in the near future, will utilize oxide films at about this critical thickness. Thinner films of oxide cannot be used in future generations of CMOS technology, no matter what their composition or state of matter. The physical thickness of the film must be kept greater than this approximate amount.
Research into new materials has suggested that a compound known as silicon nitride (nominally Si3N4) may be used for a gate dielectric with a higher dielectric constant. In a CMOS structure when the dielectric is other than silicon oxide, the layer stack of gate/dielectric/silicon is referred to as metal/insulator/silicon or MIS. The dielectric constant (xcexa) for Si3N4 is about 4.2 compared with about 2.13 for SiO2, at low frequencies. Considerable effort has already been expended on developing apparatus and methods to fabricate thin silicon nitride films. While material quality has improved, it appears that heretofore it has not been possible to avoid having the quality of this material compromised by either poor atomic structure, unsatisfactory stoichiometry (ratio of number of silicon atoms to those of nitrogen), or unwanted impurities such as oxygen or hydrogen. This has delayed the introduction of such films and now it is less clear that the modest increase in physical thickness will result in the necessary advantage for CMOS device performance in future generations.
Metal-oxide compounds of many kinds, mainly those utilizing transition metals, have potentially useful dielectric properties as well as compatibility with silicon wafers and the fabrication processes required to construct VLSI CMOS devices. There are a wide variety of materials being evaluated at present for this application, which can be grouped according to the approximate magnitude of their dielectric constant. So-called medium-xcexa materials include: Ta2O5, CeO2, TiO2, ZrO2, HfO2, and (Al,Zr)O2. (Al,Zr)O2 can be formed with many ratios of Al to Zr content (as in a metal alloy). For these, the xcexa ranges from xcx9c10 to 50, with 28 for ZrO2 and HfO2 being typical. These latter are known to provide films that at physical thickness of 50 xc3x85 function in thin-film MIS capacitors with effective (oxide-like) thickness of about 10 xc3x85. Other metal-oxide compounds are dielectrics with much larger xcexa, the so-called high-xcexa materials. Among these BaTiO3, SrTiO3, PbTiO3, ZrTiO3 and the alloys (Ba,Sr)TiO3 and (Pb,Zr)TiO3 are known to have xcexa in excess of 100 and in single crystals in excess of 1000. Film deposition and processing difficulties presently limit the introduction of these dielectrics into CMOS technology.
Oxide dielectric films on silicon of the best quality are grown at elevated temperature by reaction of environmental oxygen or oxygen-containing gasses with the silicon surface, i.e., so-called thermal oxidation. All of the dielectrics with larger xcexa must be deposited and this introduces several difficulties. One of these difficulties is that the metal-oxide compounds that compose these dielectrics will themselves react with the silicon forming a compound of metal, silicon, and oxygen in a thin layer at the interface (H. Ono and K.-I. Koyanagi in xe2x80x9cFormation of Silicon-Oxide Layers at the Interface Between Tantalum Oxide and Silicon Substratexe2x80x9d, Applied Physics Letters, Vol. 75, pp. 3521-3523 (1999)). These interfacial layers often are poor quality dielectrics with either reduced xcexa, are somewhat conducting or have a high density of charge trapping sites. Also, any appreciable thickness of this layer will then reduce the overall effective dielectric effect of the capacitor in the CMOS device and hence the deposited film must be reduced in thickness to achieve the required capacitance, and this in turn counters the intent of using the higher-xcexa film material.
One nearly ideal construction for metal-oxide dielectric films on silicon is to have a single atomic layer (i.e., a monolayer) of some suitable element terminating the silicon at the interface. Hydrogen and nitrogen are two known suitable elements for silicon termination. Silicon wafers, prior to coverage with a film, are known to be extremely reactive with oxygen containing gasses, even in trace amounts. This reaction forms a very thin oxide film, often called a native oxide on the silicon surface, but it is of poor dielectric quality. Methods are known to use aqueous solutions with HF that etch off oxides from silicon and leave behind a monolayer of hydrogen, so-called hydrogen termination (H-termination) of the surface. Connell et al (U.S. Pat. No. 5,173,474, Silicon Substrate Having an Epitaxial Superconducting Layer Thereon and Method of Making Same, (1992)) have described such a method that can be incorporated into the process for thin film deposition by the methods of physical-vapor deposition (PVD) including pulsed-laser deposition (PLD). Heating the silicon wafer to well above 400xc2x0 C. in the deposition system just prior to initiation of the flux of material to be deposited, causes the hydrogen to desorb from the silicon and to move into the chamber vacuum. Thus, hydrogen is not present on the surface during the film deposition, and consequently, upon completion of the deposition, no hydrogen remains as an interfacial layer. Yttria-stabilized zirconia (YSZ) films deposited by this method were found to form dielectrics useful in MIS capacitors (see E. Ajimine, et al. in xe2x80x9cElectrical Characterization of Metal-Insulator Semiconductor Diodes Fabricated from Laser-Ablated YBa2Cu3O7xe2x88x92xcex4/Yttria-Stabilized Zirconia Films on Si Substratesxe2x80x9d, Applied Physics Letters, Vol. 59, pp. 2889-2891 (1991); and J. Qiao, et al. in xe2x80x9cThermally Activated Reversible Threshold Shifts in YBa2Cu3O7xe2x88x92xcex4/Yttria-Stabilized Zirconia/Si Capacitorsxe2x80x9d, Applied Physics Letters, Vol. 61, pp. 3184-3186 (1992); and J. Qiao, et al. in xe2x80x9cDetermination of the Density of Trap States at Y2O3-Stabilized ZrO2/Si Interface of YBa2Cu3O7xe2x88x92xcex4/Y2O3-Stabilized ZrO2/Si Capacitorsxe2x80x9d, Applied Physics Letters, Vol. 64, pp. 1732-1734 (1994)).
It is known that nitrogen annealing has an improving effect on the dielectric quality of many types of grown oxide dielectrics on silicon (see H. Yang and G. Lucovsky, xe2x80x9cIntegration of Ultrathin (1.6xcx9c2.0 nm) RPECVD Oxynitride Gate Dielectrics into Dual Poly-Si Gate Submicron CMOSFETsxe2x80x9d IEDM Technical Digest, pp. 245-248 (IEEE, 1999)). Nitridation of the silicon surface prior to growth by exposure to NO or NH3 gas results in a very thin interfacial layer after deposition of metal-oxide dielectric films (see H. F. Luan, et al., xe2x80x9cHigh Quality Ta2O5 Gate Dielectrics with Tox,eq less than 10 xc3x85xe2x80x9d, IEDM Technical Digest, pp. 141-144 (IEEE, 1999)). The nitrogen-termination layer reduces the charge-trapping defects and acts as a diffusion barrier to prevent the oxygen of the overlying metal-oxide film from migrating down to the silicon where it would otherwise form a silicon oxide.
Methods of hydrogen termination of silicon that render improved interfaces with subsequently deposited thin, metal films have been described by I. Kondo et al. (in xe2x80x9cEffects of Different Pretreatments on the Surface Structure of Silicon and the Adhesion of Metal Filmsxe2x80x9d, Journal of Vacuum Science and Technology, Vol. A10, p. 3166-3170 (1992); in xe2x80x9cInterface Structure and Adhesion of Sputtered Metal Films on Silicon: The Influence of Si Surface Conditionxe2x80x9d, Journal of Vacuum Science and Technology, Vol. A11, p. 319-324 (1993)); by B. G. Demczyk et al. (in xe2x80x9cGrowth of Cu Films on Hydrogen Terminated Si(100) and Si(111) Surfacesxe2x80x9d, Journal of Applied Physics, Vol. 75, p. 1956-1961 (1994)); by E. T. Krastev et al. (in xe2x80x9cSurface Morphology and Electric Conductivity of Epitaxial Cu(100) Films Grown on H-Terminated Si(100)xe2x80x9d, Journal of Applied Physics, Vol. 79, p. 6865-6871 (1996) ); and by H. Gong et al (in xe2x80x9cHighly Oriented NiFe Soft Magnetic Films on Si Substratesxe2x80x9d, Journal of Applied Physics, Vol. 85, p. 5750-5752(1999)).
It is highly desirable to effectively form thin dielectric films which overcome the past problems associated with such thin film deposition.
It is therefore an object of this invention to provide improved methods for deposition of an improved thin dielectric film on silicon wafers.
It is a further object of this invention to improve control of the interface and interfacial layer between a semiconductor and a thin dielectric film on the semiconductor.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the various embodiments of the invention described hereinbelow. Methods are disclosed for gas-cluster ion-beam deposition of thin films on silicon wafers rendered free of native oxides by termination of the surface bonds. The present invention controls the interface between deposited thin films and monocrystalline silicon, which is critical to the film properties, and hence enhances the usefulness of such films in microelectronic and photonic applications. Hydrogen termination of the surface of silicon renders it inert to reoxidation from oxygen-containing environmental gasses, even those found as residue in vacuum systems, such as those used to deposit films. Nitrogen termination improves the interface with overlying metal-oxide thin films. The present invention describes both ex-situ and in-situ methods for termination.
Upon initiation of the film-deposition flux, much of the hydrogen is displaced from the surface and the film is formed in intimate contact with the silicon crystal surface forming a nearly ideal interface. Ion beams composed of nanoparticles or clusters of condensed gasses may be used to deposit thin films with advantages derived from the nanoparticle impact energy but without the known effects of ion mixing, implantation and roughening which occur with atomic and molecular ion-beam deposition. Advantages of this combination of deposition and initial substrate surface processing are described in detail hereinbelow. In particular, this invention is especially effective for, but not limited to, formation of metal-oxide compounds such as Ta2O5, CeO2, HfO2, TiO2, ZrO2 and AlxZr1xe2x88x92xO2 with higher dielectric constant than silicon dioxide films.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims.